A laser gyro is a ring laser in which two beams circulate independently in opposite sense. The gyroscopic response is obtained by beating the two output beams corresponding to the counter-circulating waves on a detector. The rotation produces a Sagnac phase shift per round-trip, which is the beat frequency that is measured. The ring laser can be either continuous wave (cw), which is the case for the conventional He—Ne laser gyro or pulsed. The laser gyro is a particular case of intracavity phase interferometry: in a laser cavity in which 2 pulses circulate, the physical parameter to be measured (rotation for the laser gyro) induces a phase shift Δϕ on one of the pulses, which, because of the resonance condition of the laser, is converted into a shift of the optical frequency Δω=Δϕ/τp. Here, τp is the cavity round-trip time at the phase velocity, hence the subscript p. In the case of a circular ring laser of radius R rotating at an angular velocity Ωr in its plane, the Sagnac phase shift is Δϕs/τp=kΔP/τp=ωΔP/P where P is the cavity perimeter, ω the (angular) optical frequency, k=ω/c the wave vector, and ΔP=2RΩrτp the effective cavity length difference seen by the counter-circulating waves. In the general case of intracavity phase interferometry, the response is highest for smallest cavity (P small) and short wavelength (large ω). In the particular case of the laser gyro, δP is proportional to the square of the perimeter P, hence
                    Δω        =                              ω            ⁢                                                  ⁢                                          Δ                ⁢                                                                  ⁢                P                            P                                =                                                                      4                  ⁢                  A                                                  2                  ⁢                  π                  ⁢                                                                          ⁢                  P                  ⁢                                                                          ⁢                  λ                                            ⁢                              Ω                r                                      =                          R              ⁢                                                          ⁢                              Ω                r                                                                        (        1        )            and the response is proportional to the ratio of the area A to the perimeter.
For the last 4 decades, active laser gyros have been limited to He—Ne lasers for the following reasons:
1. Gain competition: It is not possible to have two cw beams circulating in opposite direction in a laser medium that is not inhomogeneously broadened.
2. Injection locking of one beam into the oppositely circulating beam eliminates the gyro response, making a low pressure gas medium preferable
Both limitations are eliminated by using mode-locked lasers. If the short pulses circulating in opposite sense in the ring cavity deplete the gain at equal time intervals, there is no gain competition and no limitation to any gain medium. Injection locking by scattering of one beam into the other does not take place if the pulses meet in vacuum (or clean air).
Among all possible solid state lasers, fiber lasers have the advantage to be very light, and can extend over a large area, giving a large sensitivity to the laser gyro [R in Eq. (1)].
The following is background information on slow-light/fast light modification of a gyro response. It has been determined that the dispersion manipulation of passive gyro (e.g. Fiber Optics Gyro or FOG) does not lead to any improvement in performance. For the active laser gyro, however, a steep linear dispersion results in an increase in sensitivity. This enhancement in sensitivity can be at the expense of noise and dead band.